Diamond Back Moth

Pests of Field Crops in Southern Africa

DIAMOND-BACK MOTH

(Lepidoptera: Yponomeutidae)

Þ

Þ

Þ

Þ

Þ

Text Box: The cosmopolitan pest, Plutella xylostella, can cause severe damage to vegetables of the cabbage family in hot, dry weather. A serious problem is the high level of resistance it has developed to many of the conventional insecticides that are used by commercial vegetable producers, making control very difficult. The moth is delicate and small (7-8 mm long) with greyish-brown wings, which show a pattern of three pale diamond shapes when the wings are closed at rest. The inconspicuous larvae are pale green and cigar-shaped (being broadest in the middle section), about 10 mm long, and very irritable when disturbed, usually dropping down from the leaves on silken threads. An infestation may go unnoticed until considerable damage is caused, as they are so inconspicuous. TOP

Text Box: Host plants include all the brassicas, especially cabbage, but also ornamental plants of the Cruciferous family, such as alyssum, candytuft and stock, and weeds of the same family. These will play a part in allowing the survival of the pest, as well as its large complement of parasitoids and predators throughout the season. TOP

Text Box: The damage at first takes the form of small round holes when the larvae are young, leaving veins and often the upper epidermis intact, but in a heavy infestation, can occasionally end with leaves reduced to nothing more than a network of veins. The plants are most susceptible during their first few weeks of growth, since damage at this time, particularly in cabbages, can prevent the development of heads. TOP

Text Box: The tiny yellow eggs are laid singly or in small clusters of up to six along the main veins on the underside of the leaves. There are five larval instars, and the larvae remain on the lower leaf surface. Pupation takes place on the leaf, or occasionally on fallen leaves below the plant, in a delicate woven cocoon. The total length of the life cycle varies with the prevailing temperature, but can take as little as 14 days in the summer, lengthening up to a month in the winter. There are several generations per year. Generally populations tend to build up at the end of the dry season, but infestations have been recorded nearly all year round. TOP

Text Box: Farmers should put any strategy that might lessen the impact of diamond-back moth in place. This could include the use of windbreaks and blocks of non-host crops between plantings, and placing new plantings upwind of older crops. Early detection of the pest is essential as once established it can rapidly develop into a thriving population. The tendency of this pest to develop resistance has already been mentioned, and in some research laboratories resistance has even appeared to bio-pesticides such as those based on “Bt” (Bacillus thuringiensis). The situation has often been worsened by the repeated and indiscriminate use of pesticides that seem to work poorly, and of course, cabbages are difficult to spray because of the upright growth habit and very waxy surface of their leaves. In fact, the diamond-back moth has a very rich parasitoid fauna, with more than a dozen different parasitoids recorded in southern Africa, varying in level of control they achieve and in the times of year during which they predominate. There are also a number of other predators, such as lacewing larvae, bugs and beetles, which feed on this pest and on the cabbage aphid (another major pest of the cabbage family). In addition, a few bacterial and fungal diseases are the prime cause of caterpillar mortality during the wet season. Diamond-back moth can be such a serious pest, however, that the level of control that natural enemies exert is often too little or too slow to prevent damage. For this reason, insecticides are still needed, but they should be used wisely, to minimise damage to this natural enemy complex. In order to manage the resistance problem, it is important that insecticides of different groups are rotated. A number of ”conventional chemicals” have been registered for control in Zimbabwe for many years and unfortunately there are strong signs of resistance to them. These include: Organophosphates – dichlorvos, malathion, methamidophos, mevinphos and trichlorfon; Carbamates – carbofuran (granules in the transplant hole), carbaryl; Synthetic pyrethroids – deltamethrin; Other – endosulfan. These will of course be harsh on the natural enemies and most of them are very toxic to the user or consumer. Nevertheless, one of the better plans has been to use carbofuran granules into the transplanting holes – this systemic insecticide should give control of diamond-back moth, aphids and bagrada bugs, and has the advantage of at least not being indiscriminately sprayed. Some newer insecticides have now been registered to which resistance has not yet developed in Zimbabwe. These include: A “Bt” preparation based on the naturally occurring insect disease - an advantage is its “softness” on non-target insects, but it needs regular application as it is not long-lived, and application must be to the undersides of leaves, as it is purely a contact insecticide; An insect growth regulator, lufenuron, again reputed to be soft on non-targets; A third insecticide of a new group (chlorfenapyr) which is also showing great promise, but at the time of writing is not yet registered for use. In South Africa some interesting work has been done with botanical pesticides, in the hope that something might be developed for resource-poor farmers. One such product is the extract of the syringa tree (Melia azedarach), which is fairly closely related to the “neem” tree (of insecticidal fame). Extracts were made from the leaves, and treated cabbage leaves were shown to have fewer eggs laid on them, and to have less damage. However, the product has all the usual problems associated with botanical extracts. Little is known about its mammalian toxicity (it may well be high); it may be almost as harsh on natural enemies, if not on the environment, as conventional pesticides; and the active ingredient is likely to vary according to the growing conditions of the tree, making precise testing and development of dosage rates difficult. Whatever the pesticide used to control diamond-back moth, it is important that growers take note of and adhere to the recommended harvest intervals. TOP

Back to Cabbage.

Diamond back moth (7-8 mm) and larva (1cm).

Illustration of caterpillar damage on cabbage.